PHP contains file functions include, include_once, require, require_once difference summary

I. Use of grammar and introduction

1. Include ()
Syntax: include (/path/to/filename)
The Include () statement contains a file at the location where it was called. Contains a file that has the same content as the data in which the file was copied at the location of the statement.
Parentheses can be ignored when using include ().

The include () statement can be executed according to the criteria. Using the include () in a conditional statement has a strange phenomenon, which must be enclosed in a statement block brace, or surrounded by other statement bracketing.

2, Include_once ()
Syntax: include_once (filename)

The include_once () statement contains and runs the specified file during script execution. This behavior is similar to the include () statement, with the only difference being that include_once () will first determine if the file has been previously included, and if it is already included, ignore the inclusion.
Include_once () should be used for nested containment cases, to ensure that it is only included once to avoid problems such as function redefinition, variable re-assignment, and so on.

Summary: the include_once () function works the same as include, but it first verifies that the file is already included. If it is already included, Include_once is no longer executed. Otherwise, you must include the file. Except this is exactly the same as the include.

3, require ()
Syntax: require (filename)
Require () is, to a large extent, the same as include, where a template file is included in the location where the require call sits.
There are two important differences between require and include. First, the file will be included in the script that appears require, regardless of the location of the require. For example, even if the require is placed in an if statement that evaluates to FALSE, the specified file will still be included.
The second important difference is that when a require error occurs, the script stops running, and the script continues to execute with include.

4, require_once ()
Syntax: require_once (filename)
The require_once () statement contains and runs the specified file during script execution. This behavior is similar to the Require () statement, with the only difference being that require_once () will first determine if the file has been previously included, and if it is already included, ignore the inclusion.
Require_once () should be used for nested containment cases, to ensure that it is only included once to avoid problems such as function redefinition, variable re-assignment, and so on.

Summary: As the site grows larger, there may be duplicates containing certain files. This may not be a problem, but once you have modified the variables for the included files, you may not want this to happen because they are overwritten by the subsequent inclusion of the original file. There may also be another problem, which is the conflict of function names in the included files. Use require_once to solve these problems.
The Require_once function ensures that the file is included only once. After encountering require_once, the subsequent attempt to include the same file is ignored.

Ii. Summary of differences

1. The include () and require () statements differ.
The difference: The two structures are exactly the same except for how to handle failures.
The include () generates a warning that the script will continue to run.
Require () causes a fatal error and the script stops running.

In other words, if you want to stop processing a page when you encounter a lost file or encounter an error, use require (). Use include () if you want to continue processing the page when an error is encountered.
Note Before PHP 4.3.5, a syntax error in the include file does not cause the program to stop, but from this version.

2. The difference between include_once (), require_once () and include (), require ()
Include_once (), like require_once (), should be used to make sure it is included only once to avoid problems such as function redefinition, variable re-assignment, and so on, when the same file is likely to be included more than once during script execution. This is the main difference between include_once () and require_once () and include () and require ().

Iii. issues to be aware of

1. Path issues
Especially when nesting, be careful to include the path of the file.
For example, a file contains b files, b files contain c files, a,b,c files are not in the same folder, this time often easily error.
Solution: You can use the DirName (__file__) statement, which means to get the absolute path of the current script. such as: require_once (DirName (__file__). ' /config.php ');

2. Efficiency issues
Include_once (), require_once (), compared to include (), require (), is less efficient because they have to at least determine if the file is already contained. This issue has been greatly improved in the PHP5 version, but there is still a difference in efficiency.

PHP contains file functions include, include_once, require, require_once difference summary